The present disclosure relates to a sensor for detecting and measuring load pressure designed for measuring pressures exerted by an object on a support device, such as the body of a person reclining or seated on a support device such as a mattress or cushion, wherein said sensor is interposed between said object and said support device. More particularly, this type of pressure sensor is designed to be disposed under a portion of a mattress, wherein said portion of the mattress is interposed between a patient and the sensor, and said mattress is of the air-inflated cell type.
The pressure measured by said sensor varies as a function of the patient's weight and his contact surface with the mattress. The load pressure thus determined, notably in the sacral region, makes it possible in a known manner, notably by calibration, to regulate the ideal inflation pressure of the air inside the cells of said mattress, i.e., to adjust the pressure of the air inside the cells in order to obtain an ideal load bearing of the patient by the mattress in order to minimize the interface pressure exerted by the mattress on the patient's skin and thus reduce the risk of bedsores forming.
An application benefitting from the sensor disclosed herein, however, relates to the detection and measurement of pressures applied by the bodies of patients reclining or seated on medical support devices such as therapeutic mattresses, and notably support devices of the inflatable air-filled cell cushion or mattress type, in order to regulate the inflation pressure of the cells of the support device to prevent skin diseases linked to a prolonged immobilization on a bed or a couch. In medical practice, it is known that the interface pressures between the bodies of patients and their support devices constitute the main factor responsible for the development of skin complications, notably bedsores, due to periods of prolonged immobility of the patients on their beds or couches.
One of the proven techniques for preventing the formation and development of bedsores in patients consists of supporting the patients on beds comprising inflatable air-filled cell mattresses in which the air pressure is regulated relative to the morphology and weight of the patient with the aim of minimizing the interface pressures between the patient's body and the surface of the mattress. From load pressure data exerted by the patient on a given mattress in a particular area such as the sacral region, the ideal indentation profile of the patient over the entire surface of the mattress can be deduced and hence the value of the internal air pressure in the mattress can be adjusted for obtaining an ideal support of the patient on the mattress.
Measurement of the interface pressures or load pressure is notably achieved with sensors employing diverse technologies, which as a general rule are disposed under the inflatable cells of the mattresses on which the patients are reclining.
According to other application modes of a pressure sensor of this disclosure, said sensor can be used in a car or an airplane seat or even in combination with a conveyor belt designed to transport objects.
Depending on their technology, these sensors enable the determination of the “waterline,” i.e., the indentation distance of the patients' bodies on the inflatable cells of their support devices, or the pressures applied by the patients' bodies on the inflatable cells of their support devices.
As a function of the response signal of the sensor, the indentation depth or the pressure applied by the body on the support device is determined by an electronic control and regulation device and compared to predetermined set points in relation to the morphology of the patient and the mattress concerned. When the depth or the pressure calculated from the response signal of the sensor falls outside the range of set points, the electronic control and regulation device actuates inflation elements or deflation elements in order to adjust the inflation pressure of the inflatable cells of the support device for ensuring a level of support and comfort adapted to the patient's morphology and position.
As mentioned previously, nowadays there are various types of sensors that have been developed specifically for regulating the inflation pressure of inflatable-cell support devices such as therapeutic mattresses.
Particular mention may be made of the applicant's documents FR-A-2757378 and WO-A-9939613, which describe, respectively, two sensors with different technologies.
The document FR-A-2757378 describes an inflatable-cell support device comprising a control device consisting of an inductive sensor disposed under the inflatable cells of the mattress in the patient's sacral region. The inductive sensor makes it possible to measure an indentation distance of the body of a patient reclining on the support device comprising the sensor and to control the inflation elements of the cells in order to regulate the inflation pressure of the cells in relation to the indentation distance measured by the sensor. A first disadvantage of such an inductive sensor resides in its high manufacturing cost, which strongly impacts the cost of the support device itself. Furthermore, such an inductive sensor exhibits a thickness of at least 5 cm, which in turn confers a significant thickness of the support device in which it is integrated, making it more difficult to ensure adequate coverage by the safety rails installed on the frame of the support device for preventing falls. Lastly, because such an inductive sensor is an electromagnetic field sensor, it must satisfy the requirements of the electromagnetic compatibility (EMC) standards.
The document WO-A-9939613 also discloses an inflatable-cell support device for supporting a patient's body. This support device comprises a pressure sensor in turn comprising an inflatable chamber inflated to a predetermined pressure. The sensor is housed under the inflatable cells in the sacral support zone of the patient. Hence, the internal pressure of the inflatable chamber of the sensor varies according to the pressure variations inside the inflatable cells of the support device in relation to the morphology, weight, and movements of the patient reclining on the support device. Electronic elements then compare the respective pressures in the inflatable chamber of the sensor and in the inflatable cells of the support device and then control the inflation elements accordingly to regulate and adjust the inflation pressure inside the cells when the pressure comparison falls outside a predetermined range of set points. In order to ensure its sensor function, the sensor is filled with a weakly compressible fluid, such as silicone oil. As with the inductive sensors, a disadvantage of such a hydraulic sensor resides in the high manufacturing cost, which strongly impacts the cost of the support device itself. Furthermore, such a hydraulic sensor also exhibits a thickness of at least 3 cm, which in turn confers a significant thickness on the support device in which it is integrated, making it more difficult to ensure adequate coverage by the safety rails installed on the frame of the support device for preventing falls. Lastly, such a sensor of the inflatable chamber type is heavy because it contains approximately 4 kg of silicone oil, making it difficult to handle.
The main disadvantage of the various sensors and systems associated with regulating the inflation pressure of the cells in mattresses and other therapeutic support devices resides in the costs associated with their manufacture and use, which limits their utility in support devices for hospital use, particularly in wards specializing in the care and treatment of patients with extremely limited mobility and/or at high risk of developing bedsores.
Hence, there exists a technical problem consisting of designing and manufacturing a pressure sensor suitable for use in inflatable-cell support devices, and particularly in devices of therapeutic mattress or cushion type, for enabling regulation of the pressure inside the cells, wherein said sensor is less heavy, less bulky and less expensive to manufacture than prior art sensors and systems, while at the same time achieving similar or even improved performances. Such a sensor is particularly desirable for reducing the cost of these support devices and hence enabling their purchase and use in the scope of home medical care.
EP-2 031 362 describes a sensor of the FSR (Force Sensing Resistor) cell type based on the principle of measuring an impedance varying as a function of the compression force in kg/cm2 exerted on the cell. In EP-2 031 362, a specific number of characteristics for force sensing resistor cells was determined for enabling the generation of an electric signal and an appreciably linear response within a range of load pressures corresponding to a patient's weight in the range of between at least 50 and 140 kg. However, this type of FSR sensor is sensitive to the creeping of its sensitive element, namely a silver-based conductive ink, hence a periodic slackening of the load is required in order to avoid a drift or even a saturation of the response of the sensor over time. Furthermore, the measurement surface of the FSR sensor described in this patent, which sensor consists of a plurality of parallel-connected cells, is relatively small. In fact, an FSR sensor as described in EP 2 031 362 is designed in particular for regulating the internal air inflation pressure of mattress cells by a so-called “alternate inflation” process, wherein each cell is successively inflated and deflated in such a way that only every other cell is inflated, whereas the cells adjacent to each inflated cell are deflated, and vice versa successively. Hence, the FSR sensor in contact with the inflated cells is only subjected to said load pressure of the patient's body for a time limited to the duration of the inflation period.
Said FSR sensor is inexpensive yet gives good results for the application for which it is designed, namely pressure regulation for an air mattress without excessively high precision and suitable for a mattress that does not require high performances.
One aspect of the sensor disclosed herein is the provision of a novel type of higher precision pressure sensor that is not subject to the phenomenon of drift or saturation of its response over time if it is continuously subjected to a load on the one hand, and on the other hand, which makes it possible to distinguish weights of light patients, notably weights less than 40 kg or even less than 30 kg.
Capacitive sensors based on the variation of the capacitance of a condenser in relation to the distance between its conductive plates are known in the prior art. In electricity, a condenser is composed of two conductive plates separated by a so-called dielectric insulator. A flat condenser has a capacitance (C) expressed in picofarads (pF) by the formula C=∈0×∈R×S/d, where:                S is the surface of a conductive plate,        d is the distance between the conductive plates or the thickness of the dielectric,        ∈0 is the vacuum permittivity constant (8.85 pF/m),        ∈R is the dielectric constant of the dielectric material.        
If the thickness d of the dielectric is varied, the value of the capacitance varies logarithmically: C=f(1/d).                There are two types of prior art systems using capacitive sensors:        presence sensors operate on an all-or-nothing principle with a change of logic 0 or 1 in response to a variation in capacitance and possibly when a capacitance threshold is detected. In this type of application, the sensor does not require provision of an analog signal with a sensitive and adjusted response comprising at least one zone of linearity for the signal in relation to, e.g., the load exerted on the sensor. Essentially a YES/NO logic response is sought, depending on contact or the absence of contact with the sensor, and        capacitive measurement sensors used in laboratories measure the load time of the condenser (t=R×C), where R is the resistance placed in series with the condenser. In this type of application, the circuit is supplied with continuous voltage and the time needed for the voltage of the condenser to attain a known level is measured. This type of capacitive sensor is very expensive and relatively fragile, and furthermore requires frequent recalibration because of its drift over time.        
The sensor disclosed herein makes provision of a capacitive pressure sensor that is suitable from both a technical and economic standpoint for regulating the inflation of the cells in an air-filled inflatable-cell mattress on which a person is resting relative to the load detected by the sensor and notably by a single sensor placed as needed in a single zone of the mattress, notably in the sacral region. Furthermore, the apparatus disclosed herein makes provision of a sensor suitable for generating usable responses while being disposed under the mattress in order to prevent, among other things, the sensor from coming into contact with the patient, which could cause bedsores to form.
More particularly, the apparatus disclosed herein makes provision of a sensor that gives a response that is the most adapted in terms of sensitivity to the electric signal, exhibiting a zone of proportionality for the signal in relation to the load exerted on the sensor, at least for loads corresponding to patients' weights ranging from 40 to 120 kg, or even from 20 to 240 kg.
More precisely, the apparatus disclosed herein makes provision of a sensor for detecting and measuring a load pressure P applied to a support device, characterized in that it comprises at least one capacitive cell comprising a flat condenser comprising at least one layer of a compressible dielectric insulating material interposed between two layers of conductive material.
A sensor of the present invention exhibits the feature of being relatively inexpensive and less bulky, notably in terms of thickness while still exhibiting properties of high sensitivity and rapid stabilization of the measured electric signal, making it suitable for providing reliable data in the scope of an electronic control loop regulating the air pressure in an air-filled cell mattress cooperating with said sensor and with the mattress on which a person is resting, in relation to the load pressure exerted by said person on said sensor.
The compressibility of the dielectric insulating material makes it possible to vary the thickness d thereof relative to the load pressure applied on top of the dielectric layer and thus to vary the capacitance of the condenser relative to the load applied to the sensor. This condenser comprises a stack of flat layers and sheets, forming a flexible condenser capable of withstanding deformation, but preferably designed to be applied to an appreciably flat and rigid reference surface or support surface in such a way that the compression of the dielectric is essentially due to the force generated by the patient. Alternatively, the force applied may indent the sensor into the reference surface and the latter would absorb a portion of the applied force. In a use for monitoring the inflation pressure of an air cell mattress, the sensor is sometimes placed under the mattress, between the mattress and a relatively rigid box spring or within the mattress above a semi-rigid foam layer.
A sensor of the present disclosure may mainly be used to determine the weight of a person resting on a support device such as a mattress, as shall be explained hereafter. However, a sensor of the type contemplated herein can also be used to determine the patient's position on the mattress, namely seated or reclined and if need be, whether lying on his back or side. Essentially, the force applied to the sensor does not change; however, the load-bearing surface does change and hence the load pressure changes in relation to the patient's position, inducing a change in the indentation of the layer of compressible insulating material and hence a variation of the capacitance of the condenser.
In a known manner, a physical quantity such as electric capacitance cannot be used as is and must be converted into an electric signal. Inasmuch as a condenser is mainly used with alternating current, direct current cannot pass through the dielectric layer, as it is an insulator. The capacitive sensor is therefore integrated in an oscillator of which the frequency varies if the capacitance of the sensor varies. “Integrated in an oscillator” is understood to mean that said flat condenser of the present disclosure constitutes the condenser of said oscillator.
More precisely, said capacitive cell is integrated in an oscillator capable of generating a periodic electric signal, with preference being given to an oscillator of astable multivibration type generating a rectangular signal, wherein the frequency of said periodic signal varies as a function of said load pressure P, preferably in a range of between 15 and 30 kHz, more preferably with a variation of at least 45 Hz/kg and still more preferably of at least 50 Hz/kg for a weight of 0 to 80 kg applied to the surface of said flat condenser (1a).
The lower limit of 15 kHz ensures that the signal is completely inaudible, i.e., above the audible frequencies that could generate an annoying whistling, and the upper limit of 30 kHz complies with the electromagnetic compatibility (EMC) regulations and thus avoids electromagnetic interferences with the operation of other nearby electronic devices.
The capacitive sensor in the present application exhibits characteristics that enable it to deliver a signal exhibiting the following properties:                a sensitivity, making it possible to distinguish a load variation to the nearest kg in the range of 0 to 80 kg applied to a sensor surface representing approximately one-third of the surface area of the body,        a linearity of the signal up to approximately 40 kg (or a 120-kg patient) without saturation of the signal at loads greater than 40 kg, i.e., this value still generates a variation in capacitance in relation to the load, and        a rapid stabilization of the signal, wherein the stabilization of the sensor response must be obtained after at most 30 seconds, and the variation of the sensor signal must furthermore be essentially due solely to the variation of the physical quantity being measured.        
These different properties enable the employment of a single pressure sensor positioned notably in the sacral region and corresponding approximately to one-third of the surface area of the patient's body for regulating the inflation pressure of the air inside the cells of the mattress on which the patient is resting.
Various types of oscillators are familiar to the professional skilled in the art. An oscillator of astable multivibration type generating a rectangular signal is the simplest basic assembly, using only a few standard and inexpensive components. Furthermore, it is possible to obtain a rapid stabilization of the frequency, as these frequency variations essentially depend solely on the variation of the capacitance of the sensor. In addition, the rectangular signal delivered by the oscillator has the feature of being directly exploitable by the electronic logic processing circuits for the signals.
In some embodiments, however, said oscillator is coupled to a converter for converting the frequency (Hz) signal output by the oscillator to a voltage signal.
However, it is possible to use the frequency modulated signal output by the oscillator and dispense with the frequency/voltage conversion when the signal is processed by a microcontroller or microprocessor system.
In some contemplated embodiments, said layer of compressible dielectric material exhibits a thickness of 0.3 to 1 mm, with preference being given to 0.5 mm±20%.
As the capacitance variance is expressed as 1/d, too great a thickness d (greater than 1 mm) would result in a condenser of very weak capacitance subject to interference from other condensers in the environment, such as the electric capacitance of the patient himself. Conversely, insufficient thickness of the dielectric would lead to saturations of the sensor due to crushing of the dielectric insulating layer, and in this case, the response would become constant above a certain applied load.
A good compromise is obtained when the layer of dielectric insulating material consists of a solid elastomer sheet, preferably a synthetic elastomer such as silicone, with a Shore hardness of 45 to 55 Sh, preferably 50 Sh±5%. A Shore hardness outside this range would not give rise to the desired sensitivity and stability properties. If the material is too soft, it will tend to be crushed and the sensor will no longer be capable of reacting in a discriminatory manner above a certain load. Conversely, if the material is too hard, there will be no response with light loads.
The thickness and hardness characteristics of the material given above confer optimum sensitivity and stabilization to the electric signal with loads ranging from 0 to 80 kg applied to the sensor, which correspond to weights ranging from 0 to 240 kg of persons resting on the mattress.
More precisely, said conductive layers are composed of sheets of fabric comprising, at least partially, metallic threads of a non-oxidizing metal, or sheets of non-woven materials consisting of entwined fibers comprising at least a portion of fibers that are metallic fibers.
These embodiments of conductive sheets enable the provision of sheets of flexible material capable of accepting movements on a rigid surface on which said material is resting. However, the sensor in its final state should remain flexible and pliable with no memory effect, i.e., returning to its original shape when it is no longer subject to deformation, as otherwise, its electric capacitance would be affected. To this end, materials woven from metallic threads or non-woven materials composed of metallic fibers mixed with plastic threads or fibers, respectively, are known to the professional skilled in the art; they are used notably as electromagnetic shielding.
The thickness of the conductive sheets does not affect the value of the electric capacitance. However, the less the thickness, the greater the gain will be in terms of flexibility, lightness, and cost savings of the sensor of the type disclosed herein. In practice, the total thickness of said capacitive cell comprising said dielectric insulating layer between the two woven or non-woven conductive sheets can be less than 5 mm.
A fabric composed of metallic threads or a non-woven material made of metallic fibers is capable of accepting mechanical deformations and returning to its initial shape at rest, in contrast to a metal sheet made of solid material, which can only accept slight deformations. As metals are only very weakly elastic, an excessively high mechanical stress above their elastic limit induces a permanent deformation. On the other hand, due to the fact that the fibers are not interconnected in a metal fabric or in a non-woven material composed of metallic fibers, the mechanical deformations of the material can be absorbed without permanently changing its structure.
Preference is given to said conductive sheet being composed of a woven material made of threads of a non-oxidizing metal such as nickel and plastic threads, with preference being given to polyester or polypropylene. Conductive sheets of this type exhibit a resistivity of less than 1 ohm/m2.
Further preference is given to the 3 layers being applied directly against one another, without interposition of connecting layers, wherein the three layers are solidly connected to each other by means of attachment points, such as tack welds capable of keeping them in a superimposed position relative to each other.
The interposition of a connecting layer, notably an adhesive layer, would have negative impacts on the rigidity of the assembly on the one hand, and on the capacitance measurement on the other hand.
More particularly, a pressure sensor capable of regulating the inflation air pressure inside a mattress composed of air-filled cells comprises a condenser composed of a dielectric insulating layer and conductive layers having the same rectangular shape, preferably between 600 and 900 mm long and 400 and 600 mm wide, wherein the total thickness of the three superimposed layers of said flat condenser is less than 10 mm and preferably less than 5 mm.
This type of pressure sensor can be used as a single sensor disposed relative to the sacral region of the person resting on the mattress, said mattress being interposed between the sensor and said person.
The thinness of the flat condenser of the invention is particularly desirable because it is possible to integrate it under a mattress without having to provide a reservation in said mattress, the flexibility of the mattress composed of layers of air-filled cells being sufficient for absorbing the slight extra thickness induced by said flat condenser.
The apparatus contemplated herein also makes provision of a support device capable of supporting the body of a person, comprising at least one top layer composed of a plurality of inflatable air-filled cells and communicating with inflation elements, comprising a sensor of the type disclosed herein, said condenser of which is disposed under said top layer and connected to an electronic control and regulating device capable of controlling the inflation or deflation elements for filling or emptying, respectively, said air cells of said top layer in such a way that the internal inflation pressure of the air inside the cells is equal to an optimum set point pressure predetermined relative to the load pressure exerted by the body of a person resting on said top layer, measured by said sensor.
More particularly, said flat condenser is applied on top of a support layer that is more rigid than said top layer, said support layer being composed of a closed-cell foam layer with a density greater than 50 kg/m3, more preferably a polyether foam layer.
According to other possible characteristics of a support device of the type disclosed herein:                a top layer composed of a plurality of air-filled cells comprises a central zone corresponding to the sacral region of the body of a person reclining on said top layer, wherein said cells of said central zone are individualized and narrower than the adjacent cells of the head zone and the foot zone on either side of said central zone;        said top layer is supported by an air-filled bottom layer consisting of a single parallelepiped cell, said flat condenser being disposed under said bottom layer consisting of a single parallelepiped cell;        the pressure of the air inside said bottom layer is adjusted by said control and regulation device to the same regulation pressure as said top layer, and said bottom layer comprises a safety valve capable of being automatically closed by said control and regulation device for keeping said bottom layer sealed when a leak is detected in said top layer;        said flat condenser rests on a rigid support layer with a thickness less than or equal to 10 mm, preferably 5 mm, and said flat condenser appreciably extends over the entire width of the support device and with regard to a central zone of the support device corresponding to the sacral region of a person reclining on said top layer, and over a length of 400 to 600 mm in the longitudinal axis XX′ of said support device.        
Additional features, which alone or in combination with any other feature(s), such as those listed above and those listed in the claims, may comprise patentable subject matter and will become apparent to those skilled in the art upon consideration of the following detailed description of various embodiments exemplifying the best mode of carrying out the embodiments as presently perceived.